Amplifier circuit for frequency band expansion



July 8, 1941. I w. KLEEN 2,248,4?6

AMPLIFIER CIRCUIT FOR FREQUENCY BAND EXPANSION Filed March 17; 1938 I I l I l 1 0 U1 7 6 I NVENTOR WERNER KLEE/V avygg ATTORNEY Patented July 8, 1941 i] i T E D STATES FATE FFECE AMPLZFEER CIRCUIT FOR FREQUENCY BAND EXPANSION Germany Application March 17, 1938, Serial No. 195,327 In Germany April 3, 1937 1 Claim.

The invention is concerned with ways and means designed to expand the frequency transmission range or band width of an amplifier cascade, more particularly'of aperiodic resistancecoupled amplifiers in the direction of the higher frequencies.

In amplifiers which are intended for the satisfactory transmission of a frequency band the plate ohmic resistance must always be chosen so low that the capacities in shunt or parallel relation thereto will not cause any undue weakening of the high frequencies included in the frequency spectrum or band to be handled. This rule must be observed most particularly in the case of resistance-coupled amplifiers for highfidelity music reproduction, in television and in pieture-telegraphy signal amplifiers, and finally also in multi-channel transmission in which several messages are put on several carrier waves and are then fed through one and the same amplifier. If the external impedance of the various amplifier stages is chosen in accordance with these guidelines and views, it turns out to be comparatively low, and in such case the ensuing gain will be but small.

In order to give a clear understanding of the invention reference will be had to the accompanying drawing wherein Fig. 1 shows for the purpose of explaining the invention the various inherent capacities present in a resistancecoupled amplifier; Fig. 2b shows a tube utilized in the present invention, with suitable potentials impressed upon its electrodes for operating the same as a negative resistance device; 2a shows the signal control grid voltage-plate current characteristic of the tube shown in Fig. 2b, and fig. 3 is a resistance-coupled amplifier in accordance with the invention.

Referring now to Fig. l which affords a general idea regarding the capacities which are in parallel relation to the plate resistance, there is included in the plate circuit of the tube R1 the ohmic resistance Re to which the grid of the following stage R2 is coupled by means of a blocking condenser C. This condenser C in what follows may be left out of consideration seeing that, on the one hand, it is always chosen large, and since, on the other hand, it represents a series link which exercises no harmful effect upon the position of the upper limiting or cut-off frequency. In parallel relation to the resistance Ra are four capacities, to wit: (1) the output capacity Ca between filament and plate of the input or preamplifier R1; (2) the input capacity Ce between the control grid and the filament of the following tube R2; (3) the circuit capacitance Cs which comprises the distributed capacity of the resistance and the capacitances of the resistance and the circuit elements associated therewith and the ambient parts being at A. C. filament potential; and (4) the reacting capacity CZ, in other words, the capacity value with which the control grid-' plate capacity Cga of the next tube R2 acts at the input end of this tube.

In the presence of a given circuit organization, nothing can be altered so far as the capacities enumerated in (1) to (3) are concerned. In fact, even with extremely careful organization and design of the amplifier, there exists an irreducible limit for these values. According to the invention, the de-coupling capacity 02 is acted" upon by the properties of the following tube R2 in such a way that the sum total of the four capacities is diminished. The invention, in fact, is predicated upon the following considerations:

Assuming that the plate resistance Ra of tube R2 is a pure ohmic or active resistance, then the reacting capacity CZ is calculable by the following equation:

a" i CFC! W) where S is the slope of the characteristic of the tube R2 at the operating point, and R1 the in-- ternal resistance of tube R2. It will be seen that the capacity CZ may be made less than C' a and may be cut down to zero or even to a negative value, if the first term of the bracket is made negative. A formula corresponding to the relation derived before for the case of ohmic load of tube R2 may also be readily obtained for a nonohmic load, and in this instance also there exists the chance that capacity CZ may be reduced or even reversed in sign.

Now, according to this invention the tube following the coupling resistance whose parallel capacity is to be diminished is operated in such a way that it possesses a negative slope, in other words, an increase in the control potential would correspond to a decrease in the current of the plate or other output electrode. The slope may be so chosen that not only CZ will be caused to disappear, but will even be rendered negative, with the consequence that the other capacitances are canceled out totally or at least cut down to a low positive balance or residue.

A numerical example shall be cited to furnish clues and data for the conditions which are met with in practice. Suppose that the sum of the fixed capacitances is Cu+Cs+Ce=20 mmf.=2-10 farads (2) and the resultant resistance 2 farads Cz=-l6 rnmf. (5)

and a total capacity Ca+Cs+Ce+Cz=+3 mmf.

In other words, there results a six times lower weakening of the high frequencies, or, presupposing the same frequency response, a six times greater total amplification from a corresponding increase in the plate resistance.

For the production of negative slopes, various ways and means are available. For instance, in the case of tubes predicated for their operation upon current distribution, such as the dynatron, there exists a relation between potential of the current-distributing grid and the current of the positive electrode adjacent the filament which is characterized by a negative slope. Of particular advantage is the utilization of the so-called electron accumulation (formation of a virtual cathode) between the control electrode and the anode, for the reason that it is possible there to produce simultaneously with the negative slope also a comparatively high internal resistance. A suitable electron accumulation is securable with a triple-grid type of tube in which the first grid is the control grid, the second grid is impressed with a positive acceleration potential, while the third grid is maintained at filament potential or a negative potential. The power is put out at the following electrode, the anode, which is positively biased. So far as the basic idea of the scheme is concerned, it is immaterial whether between the third grid and the anode there are mounted additional grids, say, another screen grid in the case of a hexode, or a second screen grid plus a suppressor or cathode grid, in the case of a heptode.

Figs. 2a and 2b show, respectively, a measured characteristic and the circuit organization used therefor and comprising a tube predicated for its operation upon electron accumulation (virtual cathode). The tube here employed contains a cathode K, four grids GI to G4, and a plate A. Measurements were made to ascertain the dependence of the plate current Ia upon the voltage in of the first grid GI, the third grid being impressed by a fixed negative biasing potential, and the two other grids with fixed positive biasing potentials. It is shown that the plate current, with decreasing negative biasing voltage ur initially shows a rise and thereupon experiences a drop; and this corresponds to a negative slope of the characteristic. This form of the characteristic is connected with the formation of a space-charge threshold anteriorly of the third grid. This threshold arises as soon as the density of the electrons has attained a certain value; in spite of an increase in the current issuing from the filament, it will cause the electrons being driven back increasingly more strongly to the preceding positive electrode. The arising of such a virtual cathode and of the negative slope is promoted by choosing the distance between the positive second grid and the negative third grid comparatively large. It is expedient to choose the spacing between the said two electrodes greater than the distances between pairs of the other electrodes.

The scheme of connecting an amplifier stage above the input tube with negative slope offers in a great many instances this additional merit that grid and plate alternating potentials are in phase. For some purposes, especially in picture or video current amplification, a definite phase relation is made obligatory between the input and the output potential of the amplifier. Whereas in multi-stage amplifiers of the resistancecoupled or the choke-coupled kind comprising tubes with positive slope, the in-phase or phaseopposition condition between input and output voltage is predicated upon the use of an even or an odd number of stages, it is possible to use tubes with negative slope in all or part of the stages also with other stages numbers in order to secure the desired phase relations.

An exemplified embodiment of the basic idea of the invention is illustrated in Fig. 3. In the plate circuit of tube R1 is included the ohmic resistance Ra whose parallel capacity If is to be diminished. The next stage comprises a tube R2 working with electron accumulation; and the same contains a cathode K, four grids GI to G4 and an anode or plate A. The first grid GI in reference to which a negative slope is produced, serves as the control grid. This grid is impressed, on the one hand. with the alternating potential to be amplified from the resistance Ra through the coupling condenser C, and, on the other hand, with a suitable negative biasing voltage -u1 through the customary grid resistance Hg. The other electrodes are disposed in accordance with the circuit organization shown in Fig. 217.

What I claim is:

An amplifying system arranged for substantially uniform gain over a wide range of frequencies, comprising a first electron discharge device having an impedance connected in the output circuit thereof, said impedance having unwanted inherent capacity effectively in shunt therewith, and means for eliminating the effect of said inherent capacity, said means comprising a second electron discharge device having a cathode, a signal control grid next adjacent the cathode, a positive screen grid, a negative grid and an anode arranged in the order named, means for applying a more positive potential to the anode than to the screen grid, the relative potentials of the several electrodes being such that there results the formation of a virtual cathode between the positive and negative grids, a load resistor connected to said anode, said second device arranged to function as a negative resistance device, and means for coupling the output impedance of the first device to the signal control grid of the second.

WERNER KLEEN. 

